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‫صدق هللا العظيم‬
‫االسراء اية ‪58‬‬
Physiology of
Digestion and
Absorption
By
Dr. Abdel-aziz M. Hussein
Lecturer of Medical Physiology
Overview of the Digestive System
The Digestive System Consists of ;
a) Long hollow muscular tube or canal or
tract called gastrointestinal tract or (GIT):
it is about 5 meters long
b) Accessory glands: include:
• Salivary glands
• Liver and gall bladder
• Pancreas
Overview of the Digestive Tract
GIT consists of;
•
•
•
•
•
•
•
•
Oral cavity or mouth
Pharynx
Esophagus
Stomach
Small intestine
Large intestine
Rectum
Anus
Components of Digestive System and Their Functions
Main Functions of Digestive System
• 4 major activities of GI tract
1. Motility
• Propel ingested food from mouth toward rectum
2. Secretion of juices e.g. saliva
• Aid in digestion and absorption
3. Digestion
• Food broken down into absorbable molecules
4. Absorption
• Nutrients, electrolytes, and water are absorbed
or transported from lumen of GIT to blood
stream
Motor Functions (Motility) of
GIT
Motility of the GIT
1. Motility in the mouth
2 types;
a) Chewing is important because
-The food is lubricated by being mixed with
saliva
-The food is exposed to salivary amylase
enzyme, which begins digestion of starch
-It breaks the food into small pieces to be easily
swallowed
b) Swallowing is the transport of food from
mouth to stomach
• It consists of 3 phases or steps;
Steps of swallowing
•Buccal Phase:
Steps of swallowing
•Pharyngeal Phase:
Steps of swallowing
•Oesophageal Phase:
Motility of GIT
2. Motility of Esophagus
• The esophagus prevents air from
entering the GI tract through the
function of the upper esophageal
sphincter.
• The esophagus prevents GI
contents from re-entering the
esophagus from the stomach
through the function of the lower
esophageal sphincter.
• The contraction that sweeps
down the esophagus (primary and
secondary esophageal
peristalsis)
Motility of GIT
3. Motility of Stomach
• The stomach is divided into proximal
and distal areas.
• Proximal area is thin walled, holds
large volumes of food (to store food)
because of receptive relaxation, and
contracts weakly and infrequently.
• Distal area is thick walled with strong
and frequent contractions that mix
and propel food into the duodenum.
• Also, distal area is responsible for
gastric emptying into duodenum
Motility of GIT
3. Motility of stomach
Gastric peristalsis
Arrow showing partial liquid chyme pushed into duodenum via pylorus and
more chyme pushed forth and back in stomach cavity being small pieces
Motility of GIT
1
3
2
Motility of GIT
4. Motility of Small intestine
• Motility of the small intestine serves
four functions:
- Mixing contents with enzymes and
other secretions.
- Further reduction in particle size.
- Maximizing exposure of the
contents to membranes of intestinal
cells for absorption and digestion.
- Propulsion of contents into the large
intestine.
• Two basic motility patterns exist:
segmentation and peristalsis.
Motility of GIT
Segmentation movements
and cutting
1 ~ 5 cm
Motility of GIT
Peristalsis
Orad
caudad
Motility of GIT
5. Motility of Large intestine or colon
• 2 basic motility patterns:
a) Segmentation in the large intestine
causes the contents to be
continuously mixed
b) Mass movement propels the
contents of one segment of the large
intestine into the next downstream
segment.
• Defecation involves involuntary
reflexes and voluntary reflexes
Secretory Functions
(Secretions) of GIT
Secretions of GIT
• GI secretions function to lubricate (water and
mucus), protect (mucus), sterilize (HCl),
neutralize (HCO3-), and digest (enzymes).
• Secretions arise from specialized cells lining the
GI tract, the pancreas, liver and gallbladder.
• The total volume of GIT secretions is about 6-8
L/day
Secretions of GIT in Mouth
Salivary Glands
• Three pairs of glands
• Parotid
• Sublingual
• Submandibular
• Functions of saliva
• Lubricates, cleanses
oral cavity
• Dissolves chemicals
• Suppresses bacterial
growth
• Digest starch by
amylase
GIT secretions in Stomach
Function of hydrochloric acid
1. Activating pepsinogen
2. Provide optimum for pH for action of pepsins
3. Food protein denaturation and easy decomposition；
4. Kill bacteria in food into the stomach
5. Promoting pancreatic, small intestinal and bile secretion
6. Helping Fe2+、Ca2+ absorption.
Function of pepsins
Function of pepsinogen
protein
HCl
Pepsinogen
Pepsin
pH 2-3.5
peptone
Function of mucous and intrinsic factor
Mucus secretion
• Soluble and insoluble mucus are secreted by cells of the
stomach.
• Soluble mucus mixes with the contents of the stomach and
helps to lubricate chyme.
• Insoluble mucus forms a protective barrier against the
high acidity of the stomach content.
Intrinsic Factor
• Help absorption of vitamin B12
Pancreatic Secretion
• Pancreas has 2 functions:
a) Endocrine functions: secretes insulin and glucagon from
islets of Langerhans
b) Exocrine function: secretion of pancreatic juice
• It has 2 components: aqueous and enzymatic components.
• Aqueous component (contains HCO3) is important for
neutralizing stomach acid in the duodenum so pancreatic
enzymes can function properly
• Enzymatic component is essential for the proper digestion
and absorption of carbohydrates, fats, and proteins
• Pancreatic enzymes include trypsin, chemotrypsin, lipase, and
amylase
Functions of pancreatic juice enzymes
starch
pancreatic amylase
maltose + glucose
pH 6.7-7.0
pancrelipase +
colipase
fat（Triglyceride）
oil + monoglyceride + fatty acids
pH 7.8-8.5
trypsinogen
chymotrypsinogen
Kinase, HCl,
tissue fluid
protein
trypsin
chymotrypsin
polypeptide
carboxypepti
dase
amino
acid
The Liver and Gall Bladder
Functions of the Liver
• Metabolic regulation
• Store absorbed nutrients,
vitamins
• Release nutrients as needed
• Hematological regulation
• Plasma protein production
• Remove old RBCs
• Production of bile
• Required for fat digestion and
absorption
Secretion of small intestine
• Secretion from duodenal gland and intestinal gland
• Secretory volume is 1～3L/day
• It contains inorganic ion, mucoprotein, IgA, various
enzyme, e.g. enterokinase ,etc
• Function:
• Protective effect by mucous
• Digestion by enzymes such as peptidase, sucrase, lipase
• Dilution
Secretion of large intestine
1. colonic alkaline secretion to neutralize acids produced by
intestinal bacteria
2. secretion of mucous for protection, lubrication of fecal matter
3. Vitamin B and K absorption made from bacterial flora in colon
Digestion and Absorption
Digestion and Absorption
 Digestion is a process essential for the
conversion of food into a small and simple form.
 mechanical digestion by mastication and
swallowing
 chemical digestion by enzymes
 Absorption is the process of transporting small
molecules from the lumen of the gut into blood
stream or lymphatic vessel.
Digestion and Absorption
Digestion and Absorption
• Small intestine is primary site for
digestion and absorption of
food.
• Digestion occurs in the GI lumen
by secreted enzymes and on
surface of enterocytes by
membrane-bound enzymes.
• Absorption occurs by simple
diffusion, facilitated diffusion,
active transport, endocytosis,
and paracellular transport.
• Surface area of small intestine is
greatly increased by extensive
folding and the projection of
fingerlike villi covered with
microvilli.
Intestinal Villi
Intestinal Villi
Digestion of CHO
(salivary and pancreatic)
Absorption of CHO
Enterocytes absorb glucose and galactose through an Nadependent secondary active transport process, while
fructose is absorbed by facilitated transport.
Digestion of proteins
Absorption of proteins
•The whole proteins
by endocytosis
•Amino acids and di
and tripetides by
Na-dependent 2ry
active transport
Lumen
Endocytosis
K+
Na+
Pump
Exocytosis
Digestion of fats
Cholesterol esters
Phospholipids
Cholesterol ester hydrolase
Phospholipase A2
Cholesterol
Phosphate + Fatty acids
Absorption
Lipid digestion and absorption
Large fatty
molecule Lumen
Intestinal
Epithelia
By effects of lipase, cholesterol ester
hydrolase and phospholipase A2
Emulsification
Fatty AcidsBile salts
Cholesterol
Monoglycerides
Micelles
The smooth
endoplasmic reticulum
Triglycerides
Cholesterol esters
Phospholipids
In Golgi, they are packaged into
Vessels
Chylomicra
(CM) or VLDL
particles
(On The Apoprotein B)
chylomicra or very low density
lipoprotein (VLDL) particles.
Exocytosis
Venous sysyem
Absorption of Fats in the Small Intestine
Lymph
vessel
THANKS